The invention relates to a method of controlling an operating temperature of an air compressor, the method comprising compressing by a compressor element a mixture of air and oil and supplying it to an oil separator, separating in the oil separator the air and the oil from one another, supplying oil to an oil circulating pipe for the purpose of returning it to the compressor element and supplying at least some of the oil flowing in the oil circulating pipe to cooling when necessary, and controlling the operating temperature of the compressor by the amount of oil to be supplied to cooling such that the operating temperature is as low as possible but nevertheless so high that no condensation point is reached.
The invention further relates to equipment for controlling an operating temperature of an air compressor, the equipment comprising a compressor element for compressing a mixture of air and oil, an oil separator for separating the air and the oil from one another, an oil cooler for cooling the separated oil when necessary and a thermostatic valve which, on the basis of the temperature of the separated oil, is configured to direct a necessary amount of the oil to flow to the oil cooler and to a bypass pipe so as to bypass the oil cooler as necessary.
In an air compressor, air and oil are fed to a compressor element. A mixture of air and oil compressed by the compressor element is supplied to an oil reservoir. In the oil reservoir, the air and the oil are separated from one another. Compressed air separated from the oil is forwarded via an aftercooler and a water separator for utilization. The oil is supplied via an oil circulating pipe to be returned to the compressor element. When necessary, at least some of the oil flowing in the oil circulating pipe is supplied to an oil cooler for cooling. The oil cooler may be bypassed by a bypass pipe. Typically, an air compressor is provided with a thermostatic valve which monitors the temperature of oil in the oil circulating pipe. When the temperature of the oil is lower than an operating value of the thermostatic valve, the thermostatic valve directs the oil to the bypass pipe so as to bypass the oil cooler. When, again, the temperature of the oil is sufficiently high, the thermostatic valve directs all oil via the oil cooler. A set value of the thermostatic valve has to be sufficiently high so that in all operating conditions the air contained in the oil reservoir does not reach the condensation point, since otherwise moisture condenses from the air in to the oil, which would impair the properties of the oil considerably and thus cause damage to the entire compressor system. This, in turn, means that the operating temperature has to be kept quite high, which again stresses the mechanical strength of the air compressor as well as also contributes to impairing the properties of the oil.
U.S. Pat. No. 4,431,390 discloses a solution wherein in addition to a thermostatic valve, a bypass valve is also provided for the purpose of bypassing the oil cooler. According to the publication, values influencing the condensation of water are measured and, on the basis thereof, the pneumatically operated bypass valve is controlled to open and close the bypass pipe. With such a solution, it is in practice impossible to continuously control the operating temperature of the oil compressor since the solution only comprises switching the cooler on and off. Further, it is impossible with this solution to react to rapid variations in the load of the compressor element, which may lead to great variations in the operating temperature and air pressure such that in connection with rapid variations temperature and condensation point peaks may occur.
EP 1 937 977 discloses a solution wherein the amount of oil being supplied to cooling and the bypass pipe is controlled by a mixing valve controlled by a control device. The control device is provided with a control algorithm having the outside temperature, air pressure and environmental relative humidity inputted thereto. The purpose of the control algorithm is to calculate the lowest possible operating temperature at which no water is condensed in to the oil, and the mixing valve is controlled in an attempt to restrain impairment of the oil and to avoid condensation of water in to the oil. However, such equipment has a complex, expensive and high-maintenance structure. The controlling element is quite large. The power demand of the controlling element is also relatively high. Furthermore, it is quite challenging to make the compressor unit operate in a reliable manner in connection with a failure of the control system.